Light-emitting diodes (LEDs) can be used in thin illumination devices that provide backlighting, for instance, in liquid crystal displays (LCD) and luminaires. Thin illumination devices typically comprise an array of LEDs positioned behind objects to be backlit, such as liquid crystal plates, advertising screens or decorative tiles. A disadvantage is that LEDs emit light from a small surface area; hence the device must have a relatively thick structure in order to achieve an acceptable color and luminance uniformity. Conventional LED backlighting devices usually need more than 10 mm and additional diffuser layers in order to obtain a sufficiently uniform luminance.